1. Field of the Invention
The invention relates to a process through which it is possible to join, with the aid of laser welding, metal sheets that are pre-protected against the corrosive action of external agents; the invention is especially directed towards the field of frame and box-like structure building, used to contain electric and electronic circuits, and the aim of the present invention is to obtain effective shielding of electromagnetic fields to avoid both the spreading of external electromagnetic waves towards the inside of these frames and box-like structures, and the outward spreading from these structures and frames of electromagnetic fields produced inside them.
2. Description of the Related Art
In the fields of electro-technics, electromechanics and electronics, metal sheet containing structures, with welded together and permanently joined parts, are widely used. These metal sheets are usually made of steel and they are normally pre-protected against corrosion caused by external agents by a layer of zinc which allows the electrical contact between the sheets and the equipotentiality of distinct parts forming the final assembled containing structure. All the above creates a Faraday cage, which protects from electromagnetic fields the contents of the box-like structures obtained form cut, bent and joined sheets. Said layer of zinc must be several thousandths of millimetres thick in order to be quite resistant to external agents and to avoid that the friction caused by the bending, shaping machines should remove it, therefore uncovering the steel that should be protected in certain areas.
The electrical welding of the joined parts is carried out by passing an intense current through the metal, which heats it locally, until it reaches the temperature in which the layers of zinc fuse and on the spot where they have fused, the sheets that are pressed one on top of the other are joined. On the spots where the electrodes push the sheets one on top of the other to be welded, the surfaces become rough because the layer of zinc is burnt (it fuses at approx. 420.degree. C.) and this visible scar reduces the quality of the finished product.
Another disadvantage of electric welding in mass production is the fact that the points to be welded are not always easily to reachable by the electrodes. This is why, it is necessary to have welding machines having specifically built welding heads which involves considerable expense. Furthermore, if there are various welding points on the same steel structure, it may be necessary to provide welding machines equipped with welding heads having different shapes. Therefore, the same metal structure being built must undergo different welding passages at different stations and must pass through the action of different welding machines. These welding machines may require changing their welding heads according to the type of welding they must carry out, this is often difficult and costly.
When the aesthetic aspect is important to establish the value of the welded box, it is necessary to use non protected steel sheets, then to weld them together and finally to protect the surfaces uniformly with zinc, thus involving extra costs. In some cases it is even impossible to protect the surfaces uniformly against the corrosion of external agents, because the protecting component, paint or zinc, cannot reach the parts between the joined and welded surfaces in some points.
The inconveniences and disadvantages of the rough surfaces where they have been welded and of the welding machines with specifically designed welding heads, a consequence of the use of the electric spot welding method, may be overcome by welding with laser welding machines.
The latter ones can operate at a distance of 100 to 200 mm from the surface where the heating effect, through which the welding occurs, must take place, therefore, these machines can operate without mechanically touching the parts that need welding and without the need of an electric contact, they can carry out welding points and lines on structures that are very different or they can carry out the welding on different parts of the same structure, without needing to modify their welding head. This makes the laser welding process much faster, cheaper and more flexible, in fact the required changes to adapt the welding laser machine to different welding conditions consist in modifications of the software program that commands the movements of the robot and of its welding head and does not require mechanical modifications.
Another advantage in the use of laser welding machines consists in the fact that the welding could be carried out by hitting the internal surface (not visible) of the structure being built with the laser beam, therefore provoking a trace that will not damage the aesthetic aspect of the finished steel structure.
However, the laser welding cannot be presently used to join sheets that have been pre-protected by a thick layer of zinc, without inconveniences. In fact, in order for the welding to be accurate, the laser beam overheats the metal until it fuses at the point where the surface is hit by the said beam. Then the heat penetrates in depth until reaching the other sheet which is pressed close to the one hit by the laser beam.
In that case the reached temperatures are the ones that fuse sheets and they are however over 1000.degree. C. to allow a rapid spreading of the heat from the sheet hit by the beam to the one next to it, and/or on the points of contact, or pressed together to obtain a migration of particles from one sheet to the next obtaining a permanent joining of the parts. However, zinc burns at these temperatures and also develops violent reactions with explosions that cause perforations on the sheet hit by the laser beam and brought to a fusion temperature.
Therefore, splashes of fused metal and the formation of an irregular and rough surface on and around the perforation occur where the welding has been carried out. The roughness and unevenness of the surface hit by the laser beam make the frame unacceptable for both the aesthetic aspect and the inconsistency of the welding. This is an impediment to the use of the laser welding process, unless one uses steel sheets unprotected by zinc, which can be coated with a thick layer of zinc after the welding process, with the inconveniences described above.
In this case (using non coated sheets), the protection on the joined surfaces where the welding has been carried out, is uneven because the zinc cannot penetrate well between the overlapped sheets, therefore, the sheets will oxidise and in time ruin and damage where the parts of the structure are not well protected.
Due to all the above described facts, manufacturers that build frames and steel sheet box-like structures use electric welding, with the above described disadvantages. Another disadvantage when using sheets pre-protected by zinc is its porosity that causes the formation of prints and stains on the surfaces when touched by oily or greasy hands or machines.
Another disadvantage of the prior art techniques is that sheet frames are missing shields for electromagnetic fields corresponding to openings closed by elements that are made from non conductive material, such as connecting gaskets, plastic caps and other components that interrupt the metal continuity of the same containing frames.